1. Field of the Invention
This invention relates to the measurement of ultrasonic waves and, more particularly, to an improved interferometric technique for providing sensitive, stable and accurate measurements of ultrasonic wave intensity.
2. Description of the Prior Art
Reference is made to copending United States patent application Ser. No. 558,468, filed Mar. 14, 1975 by Mezrich et al. (which issued July 13, 1976 as U.S. Pat. No. 3,969,578), and assigned to the same assignee as the present invention, and to U.S. Pat. No. 3,716,826, issued Feb. 13, 1973 to Green. These references disclose interferometric systems for visually displaying the two-dimensional spatial distribution of the intensity of ultrasonic wave energy passing through a predetermined area of a liquid ultrasonic wave propagation medium. The interferometers employed by these systems incorporate a displaceable signal mirror and a rigid reference mirror. The displaceable signal mirror, which is situated at the predetermined area of the propagating medium, is insonified by the two-dimensional spatial distribution of ultrasonic wave energy passing through the predetermined area. The displacement of the signal mirror at any point thereof is a measure of the intensity of the ultrasonic wave insonifying that point. Thus, the spatial distribution of the displacement amplitude from point to point over the area of the surface of the signal mirror is an analog of the spatial distribution of the ultrasonic wave energy itself over the area covered thereby.
The displacement amplitudes to be measured are extremely small. For instance, the displacement amplitude of a 1.5 MHz acoustic wave of 5 nanowatts/cm.sup.2 power density is less than one picometer. Such tiny signal displacement amplitudes are many times smaller than random drift displacement due to such uncontrollable factors as air currents, thermal expansion and contraction of optical elements, etc., as well as variations from optical flatness in optical elements.
As is known, the sensitivity of an interferometer varies in a sinusoidal manner from substantially zero, when the phase difference between the two interfering output waves is either zero or .pi., to a maximum, when the phase difference between the two interfering ouput waves is either .pi./2 or 3.pi./2. Therefore, random drift displacement, discussed above, creates a problem of relatively large random change in sensitivity, which, if not solved, prevents the measurement of the relatively tiny signal displacement amplitudes.
The aforesaid U.S. Pat. application Ser. No. 558,469 solves this problem by (1) wiggling the round-trip optical path length from the interferometer reference beam by more than one-half wavelength of the interferometer coherent light at a relatively low (25 kHz) frequency compared to the ultrasonic wave frequency (1.5 MHz), and (2) peak detecting only the high frequency (ultrasonic wave frequency) component of the interferometer output over a time interval which is at least equal to one-half of a wiggling cycle. This provides a constant sensitivity for each successive wiggling half-period, despite any random drift that might have occurred during that half-period, so long as the ultrasonic wave energy persists for at least the duration of that wiggling half-period. Since the sensitivity is constant, the peak detected output signal always remains substantially proportional to the relatively tiny ultrasonic wave displacement amplitude.
The acoustic to optical image converter disclosed in the aforesaid U.S. Pat. No. 3,716,826 also wiggles the round-trip optical pathlength of an interferometer reference beam. However, in this case, the high frequency component is not peak detected (which always occurs at a point of maximum sensitivity regardless of just when during a wiggling cycle a point of maximum sensitivity occurs), but, instead, is demodulated to provide a detected output proportional to the average (rather than peak) sensitivity during the wiggling cycle. Unless the dynamic random drift which occurs during a wiggling cycle is negligible, the average sensitivity is not constant, but varies from one wiggling cycle to the next. In order to reduce this variation in sensitivity, the disclosure of U.S. Pat. No. 3,716,826 suggests employing a synchronous demodulator in which the phase of the reference beam is caused to differ by 90.degree. between each of two successive image scans of superposed fields. The display of the sum of the intensities of such successive image scans is stated by U.S. Pat. No. 3,716,826 to be a faithful analog of the acoustic field at the signal mirror.